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Using topography to estimate flood risk

This study explores the use of topography to estimate areas of flood risk. It discusses the challenges and limitations of using high-resolution topography and detailed mapping and hydrologic records. The study also presents methods for analyzing topographic data to determine floodplain areas and drainage characteristics. The findings highlight the potential of topography to predict flood risk at a low-resolution scale.

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Using topography to estimate flood risk

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  1. Using topography to estimate flood risk Brendan Murphy CE 397 Flood Forecasting May 4, 2015

  2. High-Resolution FEMA Flood Maps • Challenges: • Requires high-resolution topography • Requires detailed mapping & hydrologic records • High costs • Reality: • Global topography datasets limited to 10 - 90 m resolution • Costs are prohibitive • Not feasible for majority of areas

  3. Can topography alone be used to estimate areas of flood risk?

  4. Which topographic metrics matter? Low A High S High A Low S

  5. Travis County Topography Balcones Fault Last active ~ 15 mya USGS NED 10 m

  6. Travis County Flood Map FEMA Flood Map

  7. Methods: Data

  8. Methods: Data Lake Travis

  9. Methods: Data Area of waterbodies removed from all relevant datasets

  10. Methods: Data Balcones Fault Converted floodplain map to binary value raster at DEM resolution: 0 = not in floodplain 1 = in floodplain

  11. Methods: Data Binary raster averaged over 1 km grid Average is equivalent to fractional area of floodplain for each 1 km2 cell

  12. Methods: Data Slopes calculated from 10 m DEM then averaged over 1 km grid

  13. Methods: Data Flow Accumulation from 10 m DEM multiplied by cell area Maximum value found for 1 km grid

  14. Drainage Area Slope FEMA Floodplain Area

  15. Methods: Regressions Total of 2395 points

  16. Methods: Regressions Concern over Error Low: areas < 20% Medium: Overestimates High: Underestimates *Forcing: Predicted Area > 1 is = 1

  17. Predicted Floodplain Area

  18. Model Comparison FEMA Predicted Highland Lakes Lady Bird Lake Barton Creek Onion Creek * Biggest problem: underestimation of drainage area

  19. Model Error 58% of area within ± 10% Under Over

  20. Considerations • Expand DEM to include complete drainage areas • Improve fitting of drainage area and slope relations • Power law relationships with fractional area probably not best • Calibrate against other FEMA mapped counties Overall, topography can reasonably predict flood risk at a low-resolution scale

  21. Questions?

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